1. Field of the Invention
The present invention relates to an optical receiver that regenerates data from an optical signal based on an optimal decision threshold that is set dynamically according to the receiving power of the optical signal.
2. Description of the Related Art
With the popularization of the Internet in recent years, data traffic in communication networks has been significantly increasing. To cope with the increase of data traffic, an ultra-broadband photonic network employing a dense wavelength division multiplexing (DWDM) technology has been developed. An ultra-long-haul data communication can be performed with DWDM transmission, which uses an optical fiber including several tens of wavelength channels and a plurality of optical amplifiers connected in cascade on the optical fiber. In such ultra-long-haul data communication, however, the interference between wavelength channels significantly increases and the optical signal to noise ratio (OSNR) is seriously deteriorated due to optical noise from the optical amplifiers. Especially, data error due to the optical noise has become a bottleneck for DWDM transmission because it cannot be prevented by improving the sensitivity of an optical receiver. Therefore, to overcome this optical noise bottleneck an improvement of the error correction technology performed in the optical receiver is strongly needed.
If the optical receiver corrects the data error using forward error correction (FEC), a bit error rate (BER) of the optical receiver can be obtained from a result of the error correction. On the other hand, the receiving characteristics of the optical receiver can be improved by optimizing its decision threshold that varies depending on the OSNR or a state of chromatic dispersion due to long-haul transmission. Therefore, the performance of the optical receiver can be improved by performing a feedback control based on the BER and by adjusting the decision threshold to the optimal level.
FIG. 17 is a block diagram of a conventional optical receiver for DWDM transmission. As shown in FIG. 17, an optical receiver 1 includes a photodiode (PD) 2, a trans-impedance amplifier (TIA) functioning as a preamplifier 3, a variable-gain amplifier 4, a gain-control amplifier 5, a clock/data recovery (CDR) 6, a forward error correction (FEC) unit 7, a controller 8, and a digital-to-analog converter (DAC) 9.
The PD 2 converts an optical input signal into an electrical signal. The preamplifier 3, the variable-gain amplifier 4, and the gain-control amplifier 5 perform reshaping of the electrical signal. The CDR 6 performs regeneration and retiming of the reshaped electrical signal. The FEC 7, the controller 8, and the DAC 9 are provided to adjust the decision threshold according to the amplitude of the reshaped electrical signal as shown in FIG. 18 (see, for example, Japanese Patent Application Laid-Open No. H2-288640).
However, the optical receiver 1 needs large circuit size and its control becomes complicated because it has to perform variable-gain control to keep constant reshaped electrical signal. Furthermore, the gain of the preamplifier 3 needs to be small to prevent saturation of amplitude when the input power of optical signal increases, thereby making it difficult to improve the sensitivity of the optical receiver 1.
On the other hand, another optical receiver achieving high sensitivity with a simple configuration has also been suggested. The optical receiver includes a high-gain limiting amplifier, and a direct current (DC) feedback circuit for controlling the DC level of the positive signal and the negative signal output from the limiting amplifier. The sensitivity of the optical receiver can be improved by increasing the gain of the preamplifier, while reducing the circuit size of the optical receiver.
In such an optical receiver, however, the relation between the decision threshold of optical receiver and a feed-backed threshold control signal from an forward error correction (FEC) unit is not unique, because the condition of signal in the optical receiver greatly differs depending on, for example, the receiving power of the signal. The limiting amplifier performs a complex operation in the DC feedback control. Specifically, as long as the amplitude of an input signal is less than predetermined limiting amplitude, the limiting amplifier performs a linear operation and linearly amplifies the input signal. On the other hand, when the amplitude of the input signal reaches the limiting amplitude, the limiting amplifier performs a limiting operation and extracts a part of the input signal near cross points. The wide dynamic range of the receiving power makes it difficult to set an appropriate decision threshold, using the threshold control signal, for respective input power. As a result, a sufficient error correction cannot be achieved.